When the Zapper is fired, the game for some inscrutable reason sets the grayscale flag ($2001 D0), so instead of having white rectangles against a black background, you have white rectangles against a gray background. I do not know how responsive this game is on real hardware, with a real Zapper. It may be very unresponsive if the gray background reduces the contrast too much, or it might not matter if the light sensor somehow takes the average brightness into account, which I find a bit hard to believe.
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One means of getting this game to detect hits better in one's emulator therefore is to treat color $00 as black for Zapper detection purposes. If one wanted to patch the game, changing the byte at offset $0177 of the headered NES ROM file from $09 to $08 prevents setting the grayscale flag, so hit detection will be performed on white against black instead of white against gray.